Mixers are used in communications circuits for the purpose of generating a modulated carrier for transmission, demodulating a modulated carrier in reception, or converting a signal at some input intermediate frequency (IF) to another output radio frequency (RF) by multiplying two input signals and thereby generating a third. A number of mixer realizations, both passive and active, are known in the art, and double-balanced mixers are known particularly well due to their advantages in the suppression of unwanted spurious signals and the isolation of any one of three ports to the other two, there generally being two inputs and one output. The Gilbert Cell has been the most widely used active mixer circuit for performing the above tasks, usually incorporated within an integrated circuit. It does, however, possess certain limitations in terms of intermodulation (IM) distortion and noise figure (NF) that precludes it's use in communications systems having demanding performance specifications. The series-shunt feedback double-balanced active mixer delivers a much improved IM performance, but the lossy nature of the feedback topology does not improve the NF performance. The lossless feedback double-balanced active mixer overcomes the noise limitations of the series-shunt feedback active mixer, but still retains a significant source of IM distortion. The purpose of the present invention is to address the source of IM distortion in the lossless feedback double-balanced active mixer and significantly reduce it's impact on the mixer linearity.
Referring to FIG. 1, a schematic diagram of a lossless feedback double-balanced active mixer is shown in functional form. Here, the mixer is comprised of switching transistors 101, 102, 104, and 105, which are turned on (saturation) and off (cutoff) alternately by a differentially applied local oscillator (LO) signal. By this switching action, a pair of currents generated by driver transistors 103 and 106 are divided into four paths, there being two paths for each of two currents. The currents generated by driver transistors 103 and 106 are the result of an input intermediate frequency (IF) signal applied differentially to the input windings of a pair of feedback transformers 107 and 108. The hybrid transformers 111 and 112 combine the four currents from switching transistors 101, 102, 104, and 105, creating a differential pair of feedback currents 119 and 120, as well as an output RF signal 121. The feedback currents 119 and 120 are coupled to the output windings of feedback transformers 107 and 108, respectively, thereby forming a pair of lossless feedback amplifiers which serve to establish the conversion gain and improve the IM performance of the mixer.
Those familiar with the art will readily understand that the improved NF performance of the lossless feedback double-balanced active mixer is a result of the lack of additional noise sources in the embedding topology. This active mixer offers considerable advantages over the more traditional Gilbert Cell active mixer, especially in terms of signal-handling and performance variations over temperature due to the temperature dependency of the emitter resistance r.sub.e of the driver transistors, and the tradeoffs that are encountered in receiver and transmitter system design. It further provides substantial NF improvement over the Gilbert Cell mixer and the series-shunt feedback mixer.
Those familiar with the art will also readily understand that the IM performance of the lossless feedback double-balanced active mixer is impaired by the nonlinear emitter resistance r.sub.e of the driver transistors 103 and 106. Although this mixer offers substantial advantages in IM performance over the more traditional Gilbert cell active mixer, the presence of the nonlinear driver transistor emitter resistance causes the IM performance of the lossless feedback double-balanced active mixer to be less than ideal. This resistance is also the principal cause of conversion gain variation with temperature. It has long been desirable that a mixer, either passive or active, be available that has improved IM and temperature performance, and at the same time has an improved NF performance without the expense of added power.
It is the purpose of this invention to further advance the art of feedback mixers by addressing the primary source of IM distortion present in the lossless feedback double-balanced active mixer, and to therefore provide an active mixer of markedly improved IM performance, while at the same time conserving power consumption and retaining the NF performance and overall sense of simplicity and cost effectiveness of the lossless feedback double-balanced active mixer.